It's hit or miss. I price my items higher because most buyers will make offers. Many will send a message with a fairly low offer. There are formal offers that I believe Mercari requires to be no more than a 25% discount. They have to provide their credit card and if the offer is accepted, it gets charged immediately. Once in awhile, someone will pay full price, so I get more for my item than I was expecting. There are more messages on Mercari than on Tradesy or ebay, so it takes up more of my time, but it isn't that bad.
I find that items that are popular on ebay aren't always popular on Mercari or Tradesy, and vice versa. So it is worth it to me to list on multiple sites. I still sell the bulk of items on ebay. But I love Mercari for the quickness of getting my money. I have never had a buyer complain. About 75-80% rate me within a few hours of receiving their package, then I rate them and my money is released. The rest don't rate at all; they are probably new to Mercari and don't know the system or don't care. Then Mercari releases my money after 3 days. When I transfer money, it hits my bank in a few days.
I found it a pain when I first started using it. I like to do everything on my desktop but they didn't have that capability. I used my tablet. Now that they have the ability to list on the desktop, it is much easier for me. If I get an offer, I still have to use the app to respond. The maximum photos is 8, which sometimes isn't enough.